Still communicating with someone who is using Office 2003 or previous? You might want to include this link when sending mails with an attachment in the new Office 2007/2010/2013 document format like docx, xlsx and pptx.
By installing the Compatibility Pack in addition to Microsoft Office 2000, Office XP, or Office 2003, you will be able open, edit, and save files using the file formats new to Word, Excel, and PowerPoint 2007/2010/2013.
The Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint File Formats is no longer available for download as all the related Office versions have been out of support for quite some time now and the add-in is not needed for any currently supported version of Office.
Download: Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint File Formats